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Easy-to-grow petunias (Petunia x hybrida) are one of the most popular annual bedding plants, according to the Missouri Botanical Garden. They feature slightly fragrant, showy blooms that last from spring through the first frost, and petals come in nearly every color in the rainbow. Grown as perennials in U.S. Department of Agriculture plant hardiness zones 10 and 11, petunias have few serious disease or pest problems. If you spot bug droppings on your flowers, chances are the budworm caterpillar is to blame.

The Culprit

Tobacco budworms (Helicoverpa virescens), sometimes called geranium budworms, are the larval form of a nocturnal moth species. The small caterpillars typically have green, brown, tan or red bodies, but the exact shade varies according to the colors of the flowers they eat. Adult female moths typically lay their eggs on the petunia leaves, and the newly hatched larvae tunnel into unopened flower buds to start feeding. The voracious eaters might start munching on opened petals, leaves and stems as well. You likely won't ever see the actual caterpillars themselves, because the shy pests prefer hiding beneath the foliage or around the base of the petunia plant. Instead, you'll probably spot their tiny, pepper-like droppings, called frass, on the flowers and leaves. Other signs of budworm problems include damaged buds failing to open. Those that do open typically bear raggedy flowers with numerous holes in the petals.

Nonchemical Solutions

Monitor for budworm populations by checking the center of the petunia flowers for droppings. If you see any frass, inspect your plant again around dusk when the caterpillars become active. The easiest way to control budworm populations is to simply hand pick the caterpillars from your plant and drop them into a container of soapy water. Examine the leaves for moth eggs and wipe or hose off any that you find. Cultivating the soil beneath and around your petunia plants also helps reduce populations by exposing the caterpillars to the weather and predatory birds and insects.

Pesticide Solutions

Petunia-eating budworms are resistant to many traditional insecticides, but severe budworm infestations can often be controlled by spraying your plants with a product containing Bacillus thuringiensis (Bt). Bt is a naturally occurring soil bacterium that poisons any caterpillars that consumes it. The larvae quit feeding and typically die within two or three days. This organic, environmentally friendly ingredient might be lethal to budworms, but it won't harm beneficial insects, people or pets. Following the instructions on the product's label, mix about 4 teaspoons of Bt-based product for every 1 gallon of water in a garden sprayer. Thoroughly cover the tops and undersides of leaves. Bt works most effectively when the caterpillars are still small, so start spraying your petunias when you first notice budworm eggs or feeding damage.

Staying Safe

Before spraying your plants with any type of pesticide, put on protective clothing, including long sleeves, pants, shoes, work gloves and protective eyewear. Although not typically harmful to humans, Bt-based pesticides can still cause eye irritation if you get the mist in your face. Store unused insecticide in a cool, dry location out of the reach of children or pets.